This invention relates generally to quartz crystal resonators having a design which minimizes changes in frequency due to temperature variations. The invention is more specifically directed to a quartz crystal capable of simultaneously operating in two different modes: one mode having a relatively stable cubic frequency versus temperature characteristic and the other mode exhibiting a substantial change in frequency for temperature variations making it suitable for use as a thermometer.
This invention also contemplates a method for and an apparatus which uses such a dual mode resonator to generate an output signal having a highly stable frequency versus temperature characteristic.
It is well known that quartz crystal resonators when utilized as the frequency controlling element in an oscillator can provide better frequency versus temperature stability than when inductors, capacitors, and resistors are used as the frequency controlling elements. Despite the relatively good frequency stability offered by conventional quartz crystals such as a single mode AT-cut crystal, even greater frequency stability is required for certain applications. For example, in UHF transmitter and receiver applications where the frequency of the local oscillator is multiplied many times to yield the desired frequency, an exceptionally high level of frequency versus temperature stability is required to achieve the desired degree of stability for the derived UHF frequency.
Enhanced frequency stability has been achieved by varying an external reactance in circuit with the crystal resonator in an effort to compensate for frequency variations with temperature inherent in the crystal resonator. Typically a temperature sensor external to the quartz resonator is utilized to provide a temperature responsive signal which is utilized by appropriate compensation circuitry to minimize changes in frequency due to the frequency versus temperature characteristic of the crystal resonator. Another conventional approach to enhance stability has been to place the frequency controlling quartz resonator in a temperature controlled oven to minimize changes in temperature of the resonator.
In U.S. Pat. No. 4,039,969 for a quartz thermometer issued to Jean-Claude Martin, a single quartz crystal element includes a first set of electrodes to excite one mode which yields a relatively stable frequency and another orthogonal set of electrodes which induces a second mode having a temperature variable frequency. The difference between these two frequencies is utilized to provide a indication of the temperature.
A tuning fork type quartz crystal vibrator for low frequency oscillators is described in U.S. Pat. No. 4,320,320 issued to Eishi Momosaki et al. The quartz is cut at an angle which establishes a coupled relationship between the flexural and torsional modes of vibration of the arms to provide a favorable cubic frequency temperature characteristic. A wide range of frequency adjustments is made possible by adding weights to the vibrator. Such vibrators are used in electronic wrist watches having frequencies in the range of 100 KHZ.
In U.S. Pat. No. 3,826,931 issued to Donald L. Hammond a double rotated crystal is vibrated in two selected modes simultaneously to provide a stable output frequency which is obtained by the algebraic combination of the separate frequencies corresponding to the modes.
A temperature compensated crystal force transducer is described in U.S. Pat. No. 4,144,747 issued to Walter F. Datwyler, Jr. wherein the crystal is simultaneously resonated in two different modes. The crystal has at least one anharmonic mode having a frequency-force characteristic different from the other mode.
U.S. Pat. Nos. 4,079,280 and 4,160,183 both issued to John A. Kusters et al. are directed to the use of a double rotated quartz crystal which is operated in two different modes. The frequency-temperature deviation of one of the modes is used as an internal thermometer and the second mode as a reference frequency signal which is compensated in accordance with the difference in frequency between the two modes to minimize frequency versus temperature variations.